×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

force sewer line breaks recurrently
5

force sewer line breaks recurrently

force sewer line breaks recurrently

(OP)
The system is simple,one centrifugal pump station(2400gpm thru 4800gpm) and 2km foce main line(165psi rating pvc) which climbs 65m elevation and discharges at the top. The pump turns on/off more than 100 times a day.
And after 7 month operation, the force sewer line starts breaking recurrently in a certain area(within 150m), ring type crack at hub joint or longitudinal pipe spilt.
I suspected the surge pressure, but the actual pressure reading in the leaking area is that the static pressure during pump off  is 92psi and the pressure fluctuation at pump stop is 40psi thru 135 psi due to the surge which is less than the pipe rating. There is a surge relief valve in pump room set-up at 128psi and it works at every pump stop.
What is the cause of these pipe failures? and what is the solution?

RE: force sewer line breaks recurrently

  The vertical change in grade should cause a 92 psi increase in relative pressure.  Thus the static reading.  I am guessing a fatigue failure of the PVC pipe due to cyclic loads induced by the pump.  
   A cheap fix to try is concrete or flowable fill embedment to restrain the expansion of the PVC.  Backfill quality directly influences the strength of the pipe system. Next fix to try is stronger pipe, steel or Ductile iron.

RE: force sewer line breaks recurrently

More than a quarter century ago I think some (pre)disposition towards problems in pumped sewage systems was discussed in a paper "PVC  Pipe Performance" in the (I think maybe first) International Conference on Underground Plastic Pipe sponsored by ASCE in New Orleans, LA in 1981.  In laboratory examinations at the time of pipe samples from 332 pvc pipe failures in the UK, the authors reported the three highest categories were "laying and jointing faults", "manufacturing faults", and "operational factors".  Of the "operational factors", the most prevalent sub-cause was "surge fatigue".  A quote from that paper, "The most important operational factor found was fatigue due to surge pressure fluctuation.  This accounted for 5.7% of all failures but was found to be particularly associated with pumped sewage systems and accounted for a much higher percentage of failures in this type of application."  
While I have much more recently seen a sort of claim or inference that US-made pvc pipe is somehow better than that made abroad, some at least similar-sounding issues were apparently discussed in a subsequent report from Sandia National Labs for the United States Department of Energy in 1984, entitled “PVC (Polyvinylchloride) Pipe Reliability and Failure Modes”.   I think if you do a search of these forum archives with the “Advanced Search” feature you will also find that pvc pipe is still being sold for these conditions, and many others have unfortunately been learning these same lessons in the many years since.  I am additionally very skeptical for many reasons that a particular “fix” proposed, i.e. to make this pipe work by somehow digging up this entire plastic pipeline and encasing it with stiffer flowable fill, will work!          

RE: force sewer line breaks recurrently

Are there check valves at the pump station? Do the pumps have vfds? What size is the force main? Sounds like water hammer.   

RE: force sewer line breaks recurrently

I agree with Lburg, the kind of failures you describe are do to pressure exceeding the burst capacity of the pipe. Most large clear water pump facilites have cleverly designed systems to limit this kind of thing, while most sewage facilities don't. Most common cause of hammer when stopping is slow closing check valves. If you have external arm swing checks, put a strong spring on the arm, tending to force the valve shut. Anything to speed the closing should help.

If you have ball checks, good luck.

VFDs are nice, but power failures do happen. The piping system must be designed to stand alone with out relying on power to work.

Steve Wagner

RE: force sewer line breaks recurrently

(OP)
Let me supplement to explain the situation.
There is a check valve at pump station.
The pump doesn't have VFDs. Just on & off.
And the pipe is 16" DR25 PVC pipe.

RE: force sewer line breaks recurrently

You probably have the wrong type of check valve or an improperly adjusted check valve.

The check valve slams when the check valve is caught in a partially open position and a flow reversal occurs. It is essential that the check valve closes before a reversal occurs (such as during the pump shut down).

You need to install a check valve with a spring or lever and weight that will assist the check valve in closing quickly.

RE: force sewer line breaks recurrently

I know of a few utilities that have abandoned weighted check valves for all non-plant pumping stations and went with the spring exclusively.

I'm surprised the Utility builds stations of this size without a VFD or RVS.

Please let us know what you ended up doing and how it worked out.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources